Magnetostrictive torque sensor

ABSTRACT

Magnetostrictive wire having a low surface-to-volume ratio established by aonstant non-planar cross-section, is helically deformed into contact with a shaft under a stress maintaining the wire in fixed relation to the shaft surface. Magnetic anisotropy is imparted to the wire by twist thereof about its cross-sectional axis while the wire is being helically deformed into contact with the shaft surface to form a torque sensor through which accurate torque detection is achieved.

BACKGROUND OF THE INVENTION

This invention relates generally to detecting transmission of torquethrough shafts by use of magnetostrictive material positioned on thesurface of such shafts as disclosed in allowed copending applicationSer. No. 07/374,112, filed Jun. 21, 1989, which respect to which thepresent application is a division.

Torque sensors of the aforementioned type are generally well known inthe art as disclosed for example in U.S. Pat. Nos. 4,631,796, 4,823,617,4,765,192 and 4,823,620 to Inomata et al., Hase et al., Hase et al., andEdo et al., respectively. The shaft mounted magnetostrictive meansutilized in such torque sensors is in the form of elongated strips orribbons having a generally planar cross-sectional geometry and made ofan amorphous magnetic alloy material, the compositions of which aredisclosed, for example, in U.S. Pat. No. 4,763,030 to Clark et al. Themagnetostrictive strips are also annealed to remove mechanical strainsaccording to the disclosures in the Inomata et al. and Clark et al.patents.

The referred to prior art torque sensors, as disclosed by way of examplein the aforementioned Hase et al. and Edo et al. patents, are formedfrom magnetostrictive amorphous material wound on the shaft to bemeasured for torque within two complementary coil sections having helixangles of 45 degrees. Stationary pick-up coils disposed in inductiverelation to such magnetostrictive coil sections will respectively detecttorque applied to the shaft in either direction, in order to measuretorque in both directions. In order to obtain accurate measurement oftorque, the helically wound, magnetostrictive material must be fixed tothe surface of the shaft. Toward that end, such material was bonded byuse of suitable adhesives on the cylindrical surface of the shaft orwithin grooves formed in the shaft according to the aforementioned Haseet al. and Edo et al. patents, for example.

The aforementioned prior art torque sensors were believed to have highfigures of merit because of the good transduction properties of themagnetostrictive material based on the use of transverse field annealedamorphous magnetic alloy compositions and establishment of its fixedrelationship to the shaft by bonding thereto of generally planar stripsor ribbons of such material. However, the bonding of suchmagnetostrictive strips or ribbons to the external cylindrical surfaceof the shaft or within grooves formed therein by use of bonding adhesivehas created non-homogeneous strains sharply reducing the figure ofmerit. The application of the magnetostrictive material to the shaftsurface by sputtering to avoid the latter problem has been found to bevery difficult. Accordingly, the potential of amorphous magnetic alloymaterials in providing large values of figure of merit and lowhysteresis for torque sensors as aforementioned, has not beensatisfactorily exploited.

It is therefore an important object of the present invention to enablefull exploitation of the aforementioned properties of amorphous magneticalloy material for magnetostrictive types of torque sensors so as toavoid use of shaft mounted electronics or moving contacts found to begenerally unsatisfactory.

SUMMARY OF THE INVENTION

In accordance with the present invention, the magnetostrictive elementsof a torque sensor of the type herein before referred to are positionedon the cylindrical surface of a shaft to detect transmitted torquewithout any surface modification of the shaft and with minimal contactor use of bonding adhesive to maintain a fixed relationship between theelongated magnetostrictive elements and the external shaft surface.Toward that end, the cross sections of the elongated magnetostrictiveelements have a non-planar geometry and a surface-to-volume ratioassociated therewith to establish and maintain the requisite fixedrelationship to the external shaft surface with minimal contact underthe longitudinal stress induced in the magnetostrictive elements as aresult of its helical deformation into engagement with the shaftsurface. The magnetostricitve elements are accordingly in the form ofwire having the requisite non-planar cross-sectional geometry and asurface-to-volume ratio that is significantly less than that of thesubstantially planar types,-of magnetostrictive strips or ribbonsheretofore deemed necessary in accordance with prior art teachings.

The use of a wire type of magnetostrictive element in accordance withthe present invention also has the advantage of enabling inductivemagnetic anisotropy to be imparted therein while it is being helicallywound about the shaft by twisting of the wire about its own longitudinalaxis. Further, as a result of the present invention, a relatively modestmagnetostrictive property and a relatively small magnetic anisotropy isrequired to achieve the desired torque detection pursuant to operationalprinciples associated with the aforementioned prior art type of torquesensors. Thus, helical winding of the magnetostrictive wire on the shaftto be measured for torque as a condition induced therein may beaccomplished in a rapid and commercially feasible manner by positioninga relatively large volume of magnetostrictive material on the shaft ascompared to use of a sputtering process. The magnetostrictive wire mayalso be annealed, prior to positioning on the shaft, in a magnetic fieldpredominantly directed transverse to the axis of the wire.

These advantages, together with other objects and advantages which willbecome subsequently apparent, reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial side elevation view and an associated electricalcircuit diagram for a torque sensor in accordance with one embodiment ofthe present invention.

FIG. 2 is an enlarged partial section view taken substantially through aplane indicated by section line 2--2 in FIG. 1.

FIGS. 3 and 4 are partial section views similar to that of FIG. 2,showing different embodiments of the invention.

FIG. 5 is a partial side elevation view of a straight segment of anelongated magnetostrictive wire element associated with the torquesensor depicted in FIG. 1.

FIG. 6 is a block diagram depicting the method of making the torquesensor in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawing in detail, FIG. 1 illustrates a torquesensor constructed in accordance with one embodiment of the presentinvention, generally referred to by reference numeral 10. The torquesensor is shown associated with a shaft 12 through which torque is to betransmitted, said shaft having an external cylindrical surface 14 thatremains unaltered with the torque sensor positioned thereon.

With continued reference to FIG. 1, the torque sensor includes a pair ofcomplementary helically wound coil sections 16 and 18 positioned on theshaft 12 in fixed relationship to its external surface 14 and in axiallyspaced relationship to each other as shown. In the embodimentillustrated, each coil section is formed by closely spaced, parallelwire elements 20 helically wound at a helix angle of 45 degrees, forexample. Inductively associated with each of the coil sections 16 and 18are stationary pick-up coils 22 and 24. The coils 22 and 24 arerespectively connected through adjustable resistors 26 and 28 to asource of oscillating voltage 30. By means of the adjustable resistors26 and 28, the stationary pick-up coils 22-and 24 may be balanced toobtain a zero differential voltage applied to a torque readout circuit32 of a type generally known in the art, when no torque is beingtransmitted.

During installation of the torque sensor, as will be referred tohereinafter, shaft 12 is torqued in opposite directions in order toincrease longitudinal tension in the wires 20 of respective coilsections 16 and 18. Accordingly, when the shaft 12 is subsequentlytorqued after installation of the torque sensor, the longitudinalstresses in the wires 20 of respective coil sections 16 and 18 will varyin opposite directions to produce a differential voltage applied to thetorque readout voltage circuit 32.

As shown in FIG. 2, the magnetostrictive wire 20 is geometrically squarein cross-section so as to establish a surface-to-volume ratio that issignificantly less than that associated with the generally planar stripsor ribbons heretofore utilized for the type of torque sensors involved.Because of such cross-section of the wire 20, contact with the externalcylindrical surface 14 of the shaft established by helical winding ofthe wire about the shaft, is minimized. Furthermore, the stress inducedin the wire as a result of its helical deformation when being wound ontothe shaft is sufficient to substantially maintain the wire fixed to theshaft surface 14. If bonding adhesive is utilized, to ensure the fixedrelationship, considerably less bonding adhesive would be needed in viewof the minimal contact involved.

In accordance with one embodiment of the invention, the magnetostrictivewire is a commercially available type marketed by the Allied Corporationunder the trade name "Unitaka AF-1 and DF-1", involving an iron-boronbase composition. Of course, other iron-boron based amorphous alloymaterials may be utilized in accordance with the present invention.

FIG. 3 illustrates another embodiment of the invention in whichmagnetostrictive wire 20' for the magnetostrictive coil sections, has acircular cross-section. The surface-to-volume ratio associated with suchcircular cross-section is even less than that of the wire 20 having thesquare cross-section as shown in FIG. 2. A magnetostrictive wire 20"having an elliptical or oval cross-section as shown in FIG. 4 with asurface-to-volume ratio of a value between the ratios respectivelyassociated with the cross-sections of the wires 20 20' of FIGS. 2 and 3,may also be utilized in accordance with the present invention.

As diagrammed in FIG. 5 a straight segment of the magnetostrictive wire20 is magnetically annealed prior to helical deformation or winding inwhich the directional component 34 of the magnetic field in a directiontransverse to the longitudinal axis 36 of the wire predominates over thecomponent 38 of the field in the longitudinal direction. Such annealingof the wire 20 before its helical deformation is diagrammed in FIG. 6 asa step 40 in the making of the torque sensor in accordance with oneembodiment of the invention. After the wire is annealed in a field ofseveral thousand oersteds, for example, in the transverse direction, thewire is helically deformed into fixed contact with the external surface14 of the shaft 12 as indicated by step 42 in the diagram of FIG. 6.During such helical deformation of the wire, it is twisted about itslongitudinal axis 36 so as to impart thereto inductive magneticanisotropy as indicated by the step 46 in FIG. 6. The use of wire as theelongated magnetostrictive element made of the amorphous magnetic alloymaterial in accordance with the present invention not only establishes amore uniform cross-section of constant area but also enablessimultaneous twisting and helical deformation of the magnetostrictiveelement. The degree of twist of the wire about its own axis 36 willeffect the dynamic range and the sensitivity of the torque sensor.

During the winding of each coil section, shaft 12 is torqued in acorresponding direction so as to increase the longitudinal tension inthe wire upon removal of such torque. Subsequent application of torqueto the shaft in that direction will accordingly increase thelongitudinal tension in the wire in one coil section while decreasingthe longitudinal tension of the wire in the other coil section.Accordingly, the shaft 12 is momentarily torqued in opposite directionsduring the mounting of the sensor as denoted by the steps 48 and 50 inFIG. 6. The sensor is adjusted as indicated by step 52 before itscompletion, as denoted by reference numeral 54, by balancing the pick-upcoils 22 and 24 as indicated by step 56 in FIG. 6, involving by way ofexample, adjustment of the adjustable resistors 26 and 28 asaforementioned in connection with FIG. 1.

As will be appreciated by persons skilled in the art, the dimension ofthe wire cross-section is a factor in determining demagnetizing effectsand corresponding sensitivity of the torque sensor in addition to otherfactors such as axial spacing between the wire coils of the coilsections. Variation in the amount of active material within the wirewill vary the signal-to-noise ratio. Accordingly, depending uponoperating conditions, an optimum torque sensor arrangement may beachieved by appropriate selection of the aforementioned dimensionalfactors.

As aforementioned, the specific composition of the wire may vary as longas an amorphous magnetic material is utilized having a magnetostrictiveproperty. The magnetostriction may be of a relatively modest absolutevalue above 4 parts per million. The magnetic anisotropy induced in thewires during its helical deformation into fixed contact with the shaftmay be relatively small. While field annealing of the wire prior to itshelical deformation will substantially increase sensitivity of thetorque sensor, it is not absolutely necessary.

The use of a wire as the elongated magnetostrictive element is asignificant departure of the present invention from the prior artbecause of the unexpected benefits achieved as aforementioned,attributable to a relatively lower surface-to-volume ratio associatedwith the non-planar cross-section of the wire. In connection with themagnetostrictive wire 20 for example, having a geometrically squarecross-section, the surface-to-volume ratio is 4/√A, where A is the areaof the square cross-section. A still lower surface-to-volume ratio of2√π√A is associated with the circular cross-section of themagnetostrictive wire 20' as illustrated in FIG. 3. A surface-to-volumeratio of a value between those associated with the wires 20 and 20' isestablished by use of an oval cross-section wire 20" as shown in FIG. 4.

The foregoing is considered as illustrative only of the principles ofthe invention. Further, since numerous modifications and changes willreadily occur to those skilled in the art, it is not desired to limitthe invention to the exact construction and operation shown anddescribed, and, accordingly, all suitable modifications and equivalentsmay be resorted to, falling within the scope of the invention.

What is claimed is:
 1. In a sensor positioned on a surface of a memberfor measurement purposes, said sensor having an elongatedmagnetorestrictive element made of an amorphous magnetic materialdeformed under stress into fixed relation to the member on a contactingportion of said surface thereof, the improvement residing in saidelement having a non-planar cross-section establishing asurface-to-volume ratio preselected to minimize said contacting portionof said surface of the member during measurement thereof.
 2. Theimprovement as defined in claim 1 wherein said element is a wire, thecross-section of which encloses a substantially constant area limitingthe surface-to-volume ratio to less than 4/√A, where A is equal to saidconstant area of the cross-section.
 3. The improvement as defined inclaim 2 wherein said wire has magnetic anisotropy induced therein bysaid stress.
 4. In a sensor positioned on a shaft, the surface of whichis cylindrical, to measure torque being transmitted therethrough, saidsensor having an elongated magnetorestrictive element made of anamorphous magnetic material deformed into fixed relation to the shaft,the improvement residing in said element having a non-planarcross-section establishing a surface-to-volume ratio under which theelement is maintained fixed to a minimized external contact portion ofsaid surface of the shaft during measurement of the torque transmittedtherethrough.
 5. In a sensor positioned on an external surface of amember, an elongated helical element in fixed contact with said externalsurface, said element having magnetic anisotropy induced therein, andcircuit means inductively positioned in operative relation to theelement for measurement of stress induced in the member, said elementbeing a wire having a constant cross-sectional area in fixed contactwith the external surface of the member established under asurface-to-volume ratio for said constant cross-sectional area (A) ofless than 4√A.
 6. The sensor as defined in claim 5 wherein said circuitmeans includes a coil through which electrical current is induced as afunction of the stress induced in the member.
 7. In a sensor positionedon an external surface of a member, an elongated element in fixedcontact with said external surface, said element being made ofmagnetorestrictive material and having a constant cross-sectional areaestablishing a preselected surface-to-volume ratio, said fixed contactof the element with the external surface of the member being establishedby stress of the element on a contacting surface portion minimized bysaid preselected surface-to-volume ratio of the element.
 8. The sensoras defined in claim 7 wherein said member is a shaft to which torque isapplied.
 9. In a sensor positioned on a member, an elongated helicalelement fixed by stress therein to said member over an external contactsurface, said element being made of magnetorestrictive material andhaving a constant cross-sectional area minimizing said external contactsurface under a predetermined surface-to-volume ratio of the element andcircuit means inductively coupled to the element for measurement oftorque applied to the member.